The field of art to which the invention pertains includes internal combustion engines including use of exhaust gas charge.
Long standard in aiding control of emitted oxides of nitrogen from American manufactured internal combustion engines for automobiles has been the use of exhaust gas recirculation (EGR) in which exhaust gases are reintroduced with fresh fuel to the intake manifold during the subsequent operating cycle. For controlling quantities of recirculated gas flow over the varying parameters of engine operation, it has been common to employ an exhaust gas recirculation valve of usually the poppet or butterfly type in the recirculation path to proportionately open and close the flow passage as required.
As a practical matter, little if any exhaust gas recirculation is desired either at idle or wide open throttle (WOT) and such valves have only been approximately accurate in controlling flow quantities under operating conditions intermediate therebetween. More advanced EGR systems attempt to provide EGR during engine operation when high oxides of nitrogen levels are being produced. When oxides of nitrogen production is minimal, they attempt to eliminate EGR since the addition of EGR under those operating modes only reduces engine economy. Controls therefor have generally been responsive to a combination of engine operating parameters which are all indirect indications of oxides of nitrogen production.
In recognition of the foregoing limitations of operational accuracy, U.S. Pat. No. 3,981,283 to Kaufman proposed the use of a sonic flow metering control utilizing an axially displaceable pintle in a convergent-divergent nozzle. The pintle moves axially toward and away from the nozzle throat in order to throttle flow more or less in accordance with engine requirements, and to close the throat moves in a direction directly coinciding with the direction of incoming flow. Since sonic velocity theroretically represents maximum flow rate that can occur through a given size orifice, the patentee perceived that under conditions of sonic operation reproducible control accuracy of valve operation would thereby be enhanced.
While theoretically accurate, it has been determined that a device such as disclosed by Kaufman is unable to maintain sonic velocity at high efficiency over the entire operating range contemplated for an EGR valve which usually processes on the order of between about 60 CFM to about 1 CFM or less. With sonic operation limited to less than the required entire operating range contemplated for the valve, the perceived sonic mode effectiveness of that device is similarly limited to intermediate values only of max./min. CFM termed "turn down ratio."
Moreover, most currently available EGR valves operate in a manner whereby if failure occurs they fail in a closed mode. Closure, however, essentially eliminates the use of EGR and produces high levels of oxides of nitrogen. Since the automobile will usually achieve greater economy and better driveability in this failed mode, the faulty EGR valve is unlikely to be repaired.
Despite recognition of the foregoing problems, a ready solution has not heretofore been known.